Replace plastic trimmer capacitors C78, C88, C97, and C107 with ceramic
types. Remove excessive wax from around replacement trimmers to prevent
wax contamination. We suggest using a 12pf trimmer for C97 instead of a
7pf as listed in the service manual. This will allow you to properly adjust
the HPL lock voltage for that band. Modification: If variable resistor
R43 on the PLL unit is the 470 ohm type, replace it with a fixed-value
220 ohm version. If it's the 2.2k type, replace it with a fixed-value 1.2k
resistor.

Remarks:

While the PLL unit is lifted, we recommend soldering all joints in
the regulator section as this area runs hot. Also, inspect electrolytic
capacitors C128, C129, C132, and C137 in the regulator circuit for discoloration
caused by overheating. Replace all overheated capacitors to prevent future
failures.

Symptom:

Erratic display i.e., frequency listed will be out of the normal operating
range of the radio. Rotating the VFO knob may cause strange characters
to appear in the display. Radio does not work properly. Cycling the power
switch a few times may restore operation temporarily. Problem may be temperature
related.

Check for burned R20 (10 ohm, 1/2 watt) on logic unit. If it looks
OK, resolder cracked joints at J1 and J2 connectors on RAM unit, then reprogram.
If this does not help, then the RAM unit must be replaced. If the substitute
RAM unit does not cure the problem, there may be noise from the DC-DC converter
causing the logic unit to glitch. Follow instructions for "No frequency
display" problem.

Symptom:

No RX audio in any mode except FM. No TX. Unit is not unlocked.

Probable Cause:

PBT oscillator is not running.

Cure:

Verify absence of 9.4665 signal at W86 on main unit in 751A, or W134
in the IC-751. Replace faulty X7 on main unit in IC-751A, or X5 in the
IC-751.

Symptom:

Frequency instability in USB mode.

Probable Causes:

Bad trimmer capacitor in BFO section on main unit.

Cure:

Replace plastic trimmer capacitor with a 30pf ceramic version and realign.

Symptom:

Drastic change in audio frequency response between USB and LSB., i.e.
USB audio has too much bass while LSB audio has too much treble. TX frequency
response is also poor.

Probable Cause:

Misadjustment of the BFO or PBT oscillator, or a faulty crystal filter
FL-30 (or FL-80 in 751A) on the main unit.

Cure:

Check adjustment of the BFO and PBT oscillator. If these are OK, then
the problem is most likely in the filter.

Remarks:

If the problem is intermittent, and the radio is an IC-751 (not "A"
version), then the ground return path for the -5 supply between the display
and main units may be poor. To cure this, connect a short wire jumper between
the unused pin 2 at J2 on the display unit and chassis ground at the closest
mounting screw of the main unit PCB. If no spare pin is handy for the connector
at J2, the jumper may be soldered to the transformer shield of T1 instead.

Symptom:

AC hum or buzz in transmit and receive audio when an internal power
supply is installed.

Probable Cause:

AC is being induced into the PLL from the AC wires and power supply
case.

Cure:

Twist the AC wires that run from the rear panel to the power switch
and route them away from the PLL unit. Tape them up against the chassis
if necessary.

Remarks:

In some radios, especially older IC-751s, it is not possible to completely
eliminate all AC hum when using the internal power supply.

Replace all of these components on the RF unit: D42, D44, D45, D46,
D47, Q15. Check C174 and C179 for leakiness. There may be other failures.
Unit is not repaired until listed voltages at D47 are correct.

Symptom:

Distorted RX on strong signals. BC band RX sensitivity may be low or
marginal.

Probable Cause:

Q15 on the RF unit has become leaky.

Cure:

Check and replace Q15. If Q15 is leaky, there may be other problems
on the RF unit. See cure for RX problem listed above.

Symptom:

RX disappears when outer conductor of coax is connected to antenna
jack. Inspection reveals 6 volts DC at the center conductor of the antenna
jack.

Probable Cause:

Shorted DC-blocking capacitor C40 on the ANT SW unit.

Cure:

Replace the capacitor.

Remarks:

There are probably other problems on the RF unit if this capacitor
is bad. Check the voltages at D47 on the RF unit. Radio may have been struck
by lightning or sustained a large RF overload through the antenna jack.

Symptom:

RX cuts out randomly when turning VFO knob in IC-751A with UX-14 installed.

Probable Cause:

Incompatibility between UX-14 and 751A is causing the PLL to unlock.

Cure:

Turn off transceive mode on UX-14 (bank S1, switch #3 to OFF position).

Replace overheated C129, C132, and C137 regulator filter capacitors
on PLL unit. Inspect power supply for overheated components look OK, tighten
the screws that hold down the reg. unit. Don't forget to tighten the screw
hidden beneath the wire bundle where the DC output lines leave the case.
If none of these help, and radio is a IC-751 (not A version), then the
display DC-DC converter may need modification. Contact Icom Technical support
for update information.

Symptom:

No receiver audio at all in IC-751A. Signals can be seen on S-meter.

Probable Causes:

Cracked joint on main unit is causing no VCC the audio amp IC>

Cure:

Resolder cracked join at L46 on main unit. Repair damaged foil trace,
if necessary. Re-route ferrite beads so they will not press down on L46
(or any other components) when the top cover is reinstalled.

Symptom:

RX audio seems low in IC-751.

Probable Causes:

Unit needs update to audio amp circuitry on main unit.

Cure:

Contact Icom Technical Support for update bulletin.

Symptom:

No TX output in all modes. Problem is traced to no output at J8 on
the RF unit.

Probable Causes:

Bad Q14 amplifier on RF unit.

Cure:

Replace Q14. There may be other problems on the RF unit. Check the
DC voltages at D47.

Symptom:

Distorted/raspy sounding SSB TX. Average talk power is low, even at
higher mic gain levels. CW, RTTY and FM seem normal. RX in all modes is
OK.

Install RF decoupling capacitors on Astron power supply (if used).
If this does not help, measure voltage at bases of driver and PA transistors
in TX SSB mode. Should be around .67110 volts. 0 volts indicates trouble
and probably a bad Q6. If problem is intermittent, check for bad solder
at the legs of Q6, or D2.

Remarks:

The driver and PA idle current was set too high at the factory in some
IC-751s (not A version). Align the idle current as per the service manual
after repairs are made.

Symptom:

When using an internal power supply, the power to the radio flutters
on and off when the TX power output is increased toward maximum. Unit runs
fine when the internal power supply is substituted with an external supply.

Probable Causes:

Cracked solder in the current-sensing circuit of the power supply is
shutting it down prematurely.

Cure:

Resolder joints at the .0012 ohm resistor, R26 inside the power supply.

Remarks:

This resistor appears as a metal bar soldered to the bottom PC board.
Solder the edges of the bar BEFORE the holes at either end.

Check the value of R22 (4.7 ohms) on PA unit. (This resistor may look
OK but still be open.) Replace C18 and C25 on the PA unit if they appear
swollen or overheated. Modification for old IC-751: Install a metal film,
220-ohm 1/2 watt resistor parallel to C6 on PA unit if there isn't one
installed already. Inspect the D4 area of the ANT SW unit and replace any
burned components. If none of these solutions solve the problem, disconnect
the J10 on the RF unit and see if the problem goes away. If it does, then
there is a problem with the TX/RX switching on the RF unit. If disconnecting
J10 doesn't help, then most likely either the drivers or PA transistors
are bad, especially if the ALC meter deflection is low.

Remarks:

The spurious output may also be caused by incorrect band switch voltage
to the low-pass filter unit, or even faulty/burned components on the filter
unit itself. The above causes should be ruled out first since low-pass
filter failures are uncommon in the IC-751(A).

Symptom:

Intermittent low FM TX output power in IC-751.

Probable Causes:

Bad relay RL1 on FM unit.

Cure:

Replace bad RL1 (221-D009-M)

Symptom:

No TX output at all in RTTY or CW modes. SSB and AM are OK.

Probable Causes:

Bad carrier generator crystal on main unit.

Cure:

Replace bad X1 on main unit.

Symptom:

No frequency display, only decimal points. Problem may be intermittent
or temperature related.

Probable Causes:

Display DC-DC converter has noisy output.

Cure:

Replace all of the following dried-out electrolytic capacitors on the
display unit: C17, C18, C20, C21, C22, C23, C24, and C25. (These capacitors
are light yellow when they are new. Capacitors that have turned brown are
old and dried-out).

Remarks:

While service the DC-DC converter section, be on the look out for cracked
solder joints, as this section of the radio tends to run hot. Also, it
would be a good idea to reprogram the RAM card after servicing the DC-DC
section.

Symptom:

Poor carrier suppression in SSB mode in IC-751. Will not adjust properly.

Check station ground. If an Astron power supply is being used, add
RF decoupling capacitors to output terminals to prevent regulation problems.
If the problem occurs in an IC-751, a factory update may need to be installed
to the main unit. Contact Icom Tech Support for this information.

Symptom:

Radio seems to be stuck in "scan" mode.

Probable Causes:

Bad IC2 on the logic unit.

Cure:

Replace IC2 (RP5G01-007).

Symptom:

Loud, high pitched squeal emanates from inside radio.

Probable Causes:

DC-DC converter transformer has a loose core.

Cure:

Replace T1 (TO-9) on display unit.

Symptom:

VFO tuning is erratic, sometimes skips when changing frequencies or
bands. Display does not increment smoothly when VFO knob is turned slowly.

Do not let solvent saturate the display unit or allow it to come into
contact with the plastic display lens. Stuff a paper towel between the
rotary encoder and display unit to catch spray run-off.

Symptom:

Missing segments or mode indicators on display. Problem may be intermittent.

Probable Causes:

Bad connection between logic and display units.

Cure:

Check the connectors J5 and J13 on the logic unit where the flex cables
to the display unit are attached. If they look OK, and none of their connector
spring contacts flattened, replace the flex cables.

Symptom:

Low mic gain when using Heil mic element.

Cure:

Install modification: In 751, change value of R264 on main unit to
200k. In 751A, change value of R21 on AF-VR unit to 47k.